In Long-Term Evolution (LTE) communications networks, orthogonal frequency-division multiplexing (OFDM) modulation is employed, and a network element known as a scheduler dynamically assigns OFDM resource blocks to User Equipments (UEs) for uplink or downlink transmission. These resource blocks assignments consists of both time and frequency assignments.
With reference to FIG. 1, the smallest physical resource in LTE is called a resource element and consists of one OFDM subcarrier over the duration of one OFDM symbol. A resource block consists of 12 OFDM subcarriers over a 0.5 ms slot. The allocation of resource block is defined over Transmission Time Intervals (TTIs) of 1 ms and therefore the minimum scheduling unit is called a resource block pair which consists of 2 resource blocks. Any number of resource blocks, from 6 to 110, can be allocated to a UE. This represents a bandwidth between 1 and 20 MHz.
Typically the scheduling is performed at each TTI. To provide an optimal resource allocation, a scheduler should take into account the difference in quality among resource blocks. Indeed, each UE will have a different channel gain on different resource blocks and a resource block might be more valuable for some UEs than others.
A scheduler could aim at optimizing a single parameter, or a number of parameters, depending on the goals of an operator of the communications network. One possibility is to maximize a specific metric with respect to some constraints. For example, the scheduler may try to maximize a sum rate with some power constraint using a water-filling algorithm. In practice, the most common scheduler algorithms are:                Round-Robin Scheduler: this scheduler algorithm distributes the same number of resource blocks to all users. It is simple but it can lead to highly unfair resource allocation, where users at a cell edge are allocated the same number of resources as users located centrally in the cell, resulting in a great difference in terms of throughput.        Proportional fair scheduler: this scheduler addresses the main weakness of the Round-Robin scheduler, i.e., the fairness issue. This scheduler allocates resources to users according to priority mechanism. The priority of a user is inversely proportional to the amount of data the user could transmit in previous communication phases. That way the scheduler algorithm makes sure that all users are treated fairly in terms of throughput rather than allocated resources.        
Such a channel dependent scheduling is typically done in the downlink because it is relatively simple for a UE to measure its downlink quality. In the uplink, however, it requires the UE to transmit a sounding reference signal (SRS) to an LTE radio access node, the eNodeB with which the UE is associated. Based on the received SRS, the eNodeB will estimate the uplink quality for the UE. This method can be costly in terms of resource overhead. The frequency of the SRSs can be varied from 2 ms (for very precise quality estimation) to about 160 ms (for less precise estimation but smaller overhead).
Inter-Cell Interference Coordination (ICIC) schemes are known which apply restrictions to the radio resource management (RRM) block of the eNodeB, thus improving channel conditions across subsets of users that are severely impacted by the interference, thereby improving or attaining high spectral efficiency. This coordinated resource management can be achieved through fixed, adaptive or real-time coordination with the help of additional inter-cell signalling in which the signalling rate can vary accordingly. In general, inter-cell signalling refers to the signalling across the communication interface between neighboring cells and the received measurement message reports from UEs. In LTE networks, this interface is referred to as the X2 interface.
A problem of many current real-time ICIC schemes is that they are assuming that bases stations can communicate across the X2 inter-cell interface without significant delays. In this scenario, current and upcoming scheduling decisions from one base station/cell can be sent to neighboring cells and adaptive schemas can be used to minimize the interference.
However, current X2 interfaces between bases stations are very slow with a delay in the order of approximately 50 ms, and is only decreasing very slowly. Since this delay is much larger than the duration of a TTI, and scheduling is performed on a per-TTI basis, once current and coming scheduling decisions reach the neighboring cells, the scheduling decisions are already outdated.